Electron emitter with activator of sill
cide, boride or carbide of solid solu-
tion of barium and at least one other
alkaline earth metal



June 28, 1966 .1. H. AFFLECK Ill. err/u 3,258,636

ELECTRON EMITTER WITH ACTIVATOR OF SILICIDE, BORIDE OR CARBIDE OF SOLID SOLUTION 0F BARIUM AND AT LEAST ONE OTHER ALKALINE EARTH METAL Filed Sept. 1. 1961 INVENTORS! JOHN H. AFFLECK,I1I.

NORVAL J. HAWKINS,

THElR ATTORNEY.

United States Patent Office 3,253,636 Patented June 28, 1966 3,258,636 ELECTRON EMITTER WITH ACTIVATOR F SILI- CIDE, BORIDE 0R CARBIDE OF SOLID SOLU- TIUN 0F BARHUM AND AT LEAST ONE OTHER ALKALENE EARTH METAL John H. Aideck Ill, and Norval J. Hawkins, both of Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Sept. 1, 1961, Ser. No. 135,548 Claims. ((Cl. 313-346) This invention relates to electron emitters and pertains more particularly to new and improved thermionic emitters of the dispenser type.

Disclosed and claimed in the copending application Serial No.135,549, now Patent No. 3,176,180, of J. H. Affieck, III, filed concurrently herewith and assigned to the same assignee as the present invention is an improved dispenser-type emitter comprising a compressed fused mixture of (1) a refractory matrix material selected from the group consisting of tungsten, tantalum and molybdenum, the silicides, borides and carbides of these metals and combinations thereof; and (2) an activator compound selected from the group consisting of barium silicide, barium iodide and barium aluminide and combinations thereof. In this form of emitter barium is produced and migrates to the surface of the mixture upon thermal dissociation of the activator compound without the production or release of undesirable gases such as oxygen. Thus, no specific processing or provisions for such gases are required. Additionally, the mentioned emitter is characterized by a desirable low effective work function and a reduced evaporation rate. However, for some applications, such as use in very high vacuum electric discharge devices, it is desirable to provide dispensertype emitters with still lower activator evaporation rates. Also, it is desirable to obtain these reduced activator evaporation rates while maintaining a desirable low effective Work function and without the employment of constituents which can introduce undesirable gases into the emitter system. The present invention contemplates achievement of these desiderata through the provision of an improved dispenser system including as the activafor compound thereof a material selected from the group consisting of the silicides, borides and carbides of a solid solution of barium and one or more of the other alkaline earth metals and combinations thereof. Thus, the vapor pressure of the barium is reduced which serves to reduce effectively the evaporation rate of the activator. Additionally, in this manner the effective work function is maintained desirably low and the evaporation rate of the activator is controlled Without the employment in the system of other constituents such as reducing agents which could adversely atfect the work function or introduce undesirable gases.

Accordingly a primary object of the present invention is to provide a new and improved thermionic emitter of the dispenser type.

Another object of the present invention is to provide a new and improved thermionic emitter characterized by a desirably low effective work function and a low evaporation rate without the employment of reducing agents or other constituents which could adversely affect the work function or introduce undesirable gases.

Another object of the present invention is to provide new and improved thermionic emitters of the dispenser type which are adapted for prolonged high-density emission operation.

Another object of the present invention is to provide a new and improved thermionic emitter adapted for the production therein of barium as the activator thereof and wherein means is provided for reducing the evaporation rate of the barium.

, of the emitter.

Further objects and advantages of this invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

In carrying out the objects of the invention, there is provided a dispenser-type emitter comprising a compressed fused body of powdered materials consisting essentially of (1) a refractory matrix material selected from the group consisting of tungsten, tentalum and molybdenum, the silicides, borides and carbides of such metals and combinations thereof; and (2) an activator compound selected from the group consisting of the silicides, borides and carbides of a solid solution of (A) barium and (B) at least one of the other alkaline earth metals and combinations thereof.

For a better understanding of the present invention reference may be had to the accompanying drawing wherein is illustrated an enlarged sectional illustration of a dispenser-type emitter incorporating an embodiment of the invention.

Referring to the drawing, there is illustrated a thermionic emitter of the dispenser type generally designated 10 and constructed according to an embodiment of the invention. The emitter 16 is supported in a flared cathode holder 11 which can be formed of a refractory metal such as tungsten, tantalum and molybdenum. A heating element 12 is contained in the holder 11 for heating the member 10, thereby to effect thermal dissociation of an activator compound contained in the emitter 10 and for producing an elemental activator material to diffuse through the member 10 and migrate to an active surface 13 for providing a high density electron source at the surface 13.

The emitter 10 comprises a compressed fused body of powdered constituents of approximately 325 mesh and consisting essentially of (1) a refractory matrix material, preferably selected from the group consisting of tungsten, tantalum and molybdenum and the silicides, borides and carbides of tungsten, tantalum and molybdenum and combinations thereof; and (2) an activator compound selected from the group consisting of the;

silicides, borides and carbides of a solid solution of (A) barium and (B) at least one of the other alkaline earth metals which consist of calcium and strontium and combinations of such solid solutions.

More specifically, the emitter 10 is preferably formed v of the mentioned activator compounds such, for example,

as barium-calcium silicide (BaCa) Si However, it is to be understood that any one of the mentioned refractory materials or combinations of said refractory materials can be employed in providing the activator-containing matrix Also, the activator compound dispersed in the matrix can consist of any of the silicides, borides or carbides of a separate solution of barium and any one or more of the metals calcium and strontium. Further,-

combinations of said activator compounds can be employed such, for example, as the combination of bariumcalciurn silicide (BaCahSi, and barium-strontium-calcium silicide (BaSrCa) Si The above-discussed mixture can be formed to assume the configuration of the emitter 10 in the drawing or any other desired configuration by compressing the mixture in any suitable apparatus at approximately 70 to tons per square inch. This compression step and the apparatus employed can be generally identical to those disclosed in the above-mentioned copending Atfleck application. The compression of the mixture is such that the particles constituting the powdered constituents are fused together to a provide a coherent unitary body. In the present disclosure the term fused is used to mean that the particles constituting the body are blended together or joined as the result of the pressure applied to the mixture in much the same way that metal members can be joined or fused by the application of pressure in the method generally referred to in the art as cold welding.

After formation of the emitter 10 and mounting thereof in the holder 11, which can be carried out in a single manufacturing step, the emitter is adapted for incorporation in a vacuum electric discharge device and operation. Operation of the emitter is effected by energization of the heater 12 which heats the activator compound in the emitter 10 for effecting thermal dissociation of the compound for producing barium. Barium thusly produced fuses through the emitter body and migrates uniformly to the active surface 13 to render same emissive. The thermal dissociation of the activator compound continues throughout the life of the emitter and, thus, continually replenishes the barium provided at the active surface as an emission source.

As brought out above, the evaporation rate of the barium is reduced by employment of an activator compound comprising a solid solution of barium and one or more of the other alkaline earth metals calcium and strontium. It is believed that in an emitter system constructed according to the present invention the reduced vapor pressure of barium is explainable by Raoults Law and the realization of a more negative heat of interaction symbolized by: (AH O). Thusly, the evaporation rate of the barium is reduced by approximately 5 orders of magnitude over other systems wherein an activator compound involving a solid solution containing barium is not employed. This low evaporation rate is effective for prolonging substantially the operating life of the emitter and for insuring more uniform high density emission completely across the active surface 13 during such prolonged emitter life.

A number of emitters constructed according to the present invention as described above have been prepared and tested for the purpose of demonstrating the effective work functions and activator evaporator rates thereof. The following chart lists some of the emitter systems tested at a given operating temperature of 1250 K. and

the emission and evaporator properties observed:

Efietive Evaporation Work Rate in System in Weight Percentages Function at Grams per 1,250 K., cm. per sec. Evaporation Tungsten and Barium-Calcium Silieidc,

90% W+10% (BaCah Sin. 2. 24 8X10- Tungsten Carbide and Barium-Calcium Silicide, 90% WC+10% (BaCah Si; 2.05 1. 3Xl0- Tungsten Carbide and Barium-Calcium Silicide 90% W;C+10% (BaCah Sir 2. 28 1. 3 1O Tungsten Carbide and Barium-Strontium Silicide, 90% WC+10% (BaSr) S14 2. 27 1. fiXl- Tungsten Carbide and BariunrStrontium Silicidc, 90% W2C+10% (BaSrh Sil 2.32 1.6 10 TungstenBoride and Barium-Strontium Silicide, 90% W2B +l0% (BaSri Sit 3.00 7. 5X10- Tantalum and Barium-Calcium Silicide,

90% Ta+10% (BaCa)1 Si; 2. 32 3. 5X10- Tantalum Carbide and Barium-Calcium Silieide, 90% TaC+l0% (BaCa)1 Si4 2. 45 2. 6X10- It is to be understood that while the above chart does not list all of the systems falling within the purview of the invention the other systems encompassed by the present invention, including, for example, those involving use of compounds of solid solutions of barium and calcium or a solid solution of barium, calcium and strontium with a matrix formed of any of the above-referenced refractory metals and silicides, carbides and borides of such refractory metals are equally effective for obtaining the advantages of the present invention. Specifically, any of these alternative forms is effective for providing a low effective work function emitter with an evaporation rate for the barium which is considerably lower than where a solid solution of barium and another alkaline earth metal is not employed in providing the activator compound.

Also, while the activator compound was in each case in the systems included in the above chart approximately 10 weight percent of the composition of the system, it is to be understood that the weight percentage composition of the system can vary substantially and still be characterized by a desirably low effective work function and a substantially reduced evaporation rate. When an activator compound comprising a solid solution of barium and one other alkaline earth metal is employed it is considered preferable that each constitute approximately 50 mol percent of the compound and when barium and two other alkaline earth metals are employed it is considered preferable that each constitute about 33% of the resultant solid solution.

Additionally, in conducting the tests whereby the abovelisted information was obtained, the above-noted temperature of 1250 K. was arbitrarily selected for test purposes and the effective work functions and activator evaporation rates indicated are those calculated for this temperature. At different operating temperatures commensurately different effective work functions and activator evaporation rates are obtainable and are readily determinable by use of calculations well known to those skilled in the art.

While specific embodiments of the present invention have been described and shown, it is not desired that this invention be limited to the particular forms shown and described, and it is intended 'by the appended claims to cover all modifications within the spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A thermionic emitter of a singular integral body consisting of a refractory matrix having dispersed therein an activator compound selected from the group consisting of the silicides, borides, and carbides of a solid solution of (A) barium and (B) at least one other alkaline earth metal, said activator compound comprising approximately 10 weight percent of said body, the said solid solution within said refractory matrix constituting the sole supply of barium for said emitter during extended operation thereof.

2. A thermionic emitter comprising a body of compressed fused powdered constituents consisting essentially of (1) a refractory matrix material selected from the group consisting of tungsten, tantalum and molybdenum, and the silicides, borides and carbides of tungsten, tantalum and molybdenum and (2) an activator compound selected from the group consisting of the silicides, borides and carbides of a solid solution of (A) barium and (B) at least one of the metals selected from the group consisting of calcium and strontium, said activator compound comprising approximately 10 weight percent of said body.

3. A thermionic emitter comprising a densely packed fused matrix of powders of a refractory matrix material and an activator compound selected from the group consisting of the silicides, borides and carbides of a solid solution of barium and calcium and wherein said barium and calcium each comprises approximately 50 mol percent of said compound.

4. A thermionic emitter comprising a densely packed fused mixture of powders of the refractory matrix material and an activator compound selected from the group consisting of the silicides, borides and carbides of a solid solution of barium and strontium and wherein said barium and strontium each comprises approximately 50 mol percent of said compound.

5. A thermionic emitter comprising a densely packed fused mixture of powders of a refractory matrix material and an activator compound selected from the group consisting of the silicides, borides and carbides of a solid solution of barium, calcium and strontium and wherein said barium, calcium and strontium each comprises approximately 33 mol percent of said compound.

Reierences Cited by the Examiner UNITED STATES PATENTS 2,647,216 7/1953 Brown 313-3461 X 2,654,045 9/1953 Wright 313346 2,700,000 1/1955 Levi et a1 313346.1 X

6 2,700,118 1/1955 Hughes et a1 313-346 2,737,607 3/1956 Lemmens et a1. 313-346.1

DAVID J. GALVIN, Primary Examiner.

5 RALPH G. NILSON, GEORGE N. WESTBY,

Examiners.

C. O GARDNER, R. SEGAL, Assistant Examiners. 

1. A THERMIONIC EMITTER OF A SINGULAR INTEGRAL BODY CONSISTING OF A REFRACTORY MATRIX HAVING DISPERSED THEREIN AN ACTIVATOR COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE SLICIDES, BORIDES, AND CARBIDES OF A SOLID SOLUTION OF (A) BARIUM AND (B) AT LEAST ONE OTHER ALKALINE EARTH METAL, SAID ACTIVATOR COMPOUND COMPRISING APPROXIMATELY 10 WEIGHT PERCENT OF SAID BODY, THE SAID SOLID SOLUTION WITHIN SAID REFRACTORY MATRIX CONSTITUTING THE SOLE SUPPLY OF BARIUM FOR SAID EMITTER DURING EXTENDED OPERATION THEREOF. 